Hi, guitpic1!
Technically, using an external USB hub, you can have even more thumb drives connected to the instrument. The question is whether this is advisable or not... and the answer is a resounding NOT. Let me explain.
When the keyboard powers up, it will enumerate the USB drives, and assign a drive letter to each (although this is not shown to the user, internally Yamaha uses drive letters the same as your computer). The first external drive gets the letter I:, the second J:, and so on. With several drives, there is no guarantee that the drives will get the same drive letter assigned each time. As all file references are saved as complete paths (including the drive letter), you will find that file references will randomly work or not depending on how the keyboard assigned drive letters.
There is also a second question here: what do you want the space for? Instruments and sound samples (i.e., instrument banks created with YEM) can not be stored neither in the internal 4GB nor in external memory: there is a special dedicated memory for samples, which is connected directly to the sound generator and not accessible from anywhere in the instruments interface, which has 1Gb in the case of the SX900. This sample memory can not be expanded in any way.
The internal drive (4GB) and the external USB drives are mostly used for styles, multipads, registrations, midi files, text files, chord files and audio files (wav and mp3). And settings back-up. With the exception of Audio files, all of these are extremely small (<100kB). In most cases, a 16GB or 32GB memory stick will hold anything you want and more, and still be more than half empty! The only case where a large external drive makes sense is if you intend to do audio recordings on your keyboard, and keep them all in the drive, in which case you should look for a big and fast pen drive.
Regards!
